1. Field of the Invention
The present invention relates to an image pickup optical system suitable to pick up an image of an object (observation object) from a low angle.
2. Description of the Related Art
When an image of an object is picked up from a low angle close to an angle corresponding to a surface thereof, that is, from an oblique direction, an image of an uneven surface of the object can be picked up at high precision.
This is because, when the object is viewed from the oblique direction, the object is observed while the influences of substances composing the object and a size of unevenness are increased due to a polarization state or the like.
A tilt image pickup optical system has been known as an image pickup system suitable to perform image pickup while an entire image pickup surface tilted in an optical axis direction of an image pickup system is brought into focus. This image pickup principle was found by Scheimpflug. The optical system is also called a Scheimpflug optical system.
Up to now, an image pickup apparatus using a technique of the Scheimpflug optical system has been proposed (Japanese Patent Application Laid-Open No. H11-190864).
A projection apparatus using a projection optical system for projecting an image in an oblique direction, which is represented by a rear projector, has been known as an optical device to which the Scheimpflug optical system is applied. In particular, a projection apparatus for performing projection in an oblique direction at a maximum angle has been known in order to thin the projection apparatus (Japanese Patent Application Laid-Open No. H06-265814 (corresponding to U.S. Pat. No. 5,442,413) and Japanese Patent Application Laid-Open No. H02-079037).
Hereinafter, the Scheimpflug optical system will be briefly described.
FIG. 11 is a schematic explanatory diagram of an optical system for illustrating the Scheimpflug principle which is a tilt image pickup principle. In FIG. 11, an image pickup system 101, an image plane 102 of the image pickup system 101, and an object 120 which is a surface having a size are illustrated.
In general, when a principal plane 101a of the image pickup system 101 is parallel to the image plane 102 in which an image pickup element is located and the object 120 is tilted relative to an optical axis 101b, the entire region of the object 120 cannot be in focus at a time.
In order to make the entire region of the object 120 in focus, a structure is preferably employed in which an intersection P of an extended plane 102a extending from the image plane 102 in which the image pickup element is located and an extended plane 120a extending from the object 120 intersects with the principal plane 101a of the image pickup system 101. This is the Scheimpflug principle.
FIG. 11 illustrates a positional relationship among the object 120, the principal plane 101a of the image pickup system 101, and the image plane 102 in a case where Scheimpflug principle is actually applied to the image pickup system 101.
Assume that a plane including an axis 101c which is perpendicular to the image plane 102 and passes through a principal point of the image pickup system 101 and the optical axis 101b is a reference plane.
An angle α formed between the surface of the object 120 and the axis 101c in the reference plane is a shooting angle. Note that “a low angle” corresponds to a small shooting angle α, for example, an angle equal to or smaller than 20°.
An angle θ formed between the image plane 102 (final image plane in a case where an optical system for reimaging is used) and the principal plane 101a of the image pickup system 101 is a tilt angle.
An angle β formed between the surface of the object 120 and the image plane 102 is a Scheimpflug angle.
A shooting angle in a case where the object is shot from an oblique direction using an optical system satisfying the Scheimpflug condition is normally in a range up to approximately 20°. Many optical systems satisfying the Scheimpflug condition are not expected to perform image pickup from a shooting angle equal to or smaller than 20°.
In order to enable image pickup from the low angle corresponding to the shooting angle α equal to or smaller than 20°, the tilt angle is preferably increased with a reduction in shooting angle.
However, when the tilt angle θ is to be increased, a shooting lens diameter lengthens and the influence of aberration increases, so that design and manufacture are difficult. Therefore, an actual maximum tilt angle has been up to approximately 10°.
A rear projection optical system using the Scheimpflug optical system is originally used for oblique projection, not for image pickup on a subject tilted relative to the optical axis, so an angle formed between a final projection plane and an optical path of a projection light beam is large. Therefore, when an image pickup optical system in which a central light beam is incident on the image plane nearly perpendicularly (described later) is used, it is difficult to obtain an image in focus in a wide area of the subject.